The present invention relates generally to beam-forming illumination systems, and more specifically to those with sufficiently narrow solid angles for the output beam to be called collimators. Examples are flashlights and searchlights. The primary limitation upon their performance is étendue-invariance, by which the angular radius θ of the beam is determined by the ratio of source diameter d to aperture diameter D: sin θ=d/D, when the source radiates into a hemisphere. The beamwidth is twice the angular radius and usually is defined as full width at half maximum (FWHM).
Light emitting diodes (LEDs) are an example of such a hemispheric source. Their small size and ever-improving luminous efficacy are propelling them into market predominance in many fields of lighting. Since their hemispheric emission is too wide for most lighting tasks, LEDs are installed in luminaires that generate narrower output angles. So far, LED flashlights are gaining market prominence, and LED downlights are being installed in ceiling receptacles. Automotive headlights in particular are and example of a field where market pressure for device compactness collides with the étendue theorem. The present luminaires can augment LED brightness, and some embodiments can achieve aperture widths only half the étendue-limited size, with only a modest sacrifice in overall output flux.